The present invention relates to a method and apparatus for detecting the ingress of liquid into an elongate hollow structure, at least part of which is submerged in a liquid, and more particularly to such apparatus suitable for incorporation in offshore installations.
Many industrial applications require means of detecting the ingress of liquid into a structure and such means can be effectively used to detect leaks. An example of such an application can be found in the oil and gas industry. Here, offshore platforms are supported by water or oil-filled hollow members extending from the sea bed and braced by air filled hollow cross members. As the cross members are air filled, the presence of water within a cross member provides an indication that damage has occurred. It is most important that such damage to the cross members is detected as soon as possible to reduce the risks of injury to people or property.
One solution to the monitoring problem is to use a flooded member detection (FMD) method. This monitoring can either be performed using divers or remotely operated vehicles (ROVs). In each case detection techniques may involve the use an ultrasonic technique or a gamma radiation technique. In each of these methods, a detector identifies the presence of water directly behind an area of metal under examination. If this monitoring detects water more detailed examination of the member can be carried out to determine the necessary remedial action.
This method has a number of disadvantages. One such disadvantage is that in parts of the structure inaccessible to the diver or the ROV monitoring is not possible. Another is that by its nature the process can only take place at inspection intervals, typically every two years. If deterioration of a member occurs shortly after inspection it may be some considerable time before the same member is re-examined to enable the detection of damage, leading to a compromise in safety. The method is also limited to only finding water directly behind the point currently under examination, which means that inspection must be carried out at or adjacent the lowest point of the structure. This method is expensive to carry out with costs for the hire of an appropriate ROV or diving team being in the region £100,000 per day.
The present invention seeks to obviate or mitigate some or all of the problems identified above. In particular, it is an object of the present invention to provide a method of detecting damage to a structure which does not require direct access to the structure under investigation and which detects damage as soon as it occurs.